Diminish overwrites all previous effects that set the target creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Diminish resolves will overwrite this effect.

11/17/2017

Effects that modify the target creature’s power or toughness, such as the effects of Elesh Norn, Grand Cenobite or prowess, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters).